Computer systems, video games, electronic appliances, digital cameras, and myriad other electronic devices include memory for storing data related to the use and operation of the device. A variety of different memory types are utilized in these devices, such as read only memory (ROM), dynamic random access memory (DRAM), static random access memory (SRAM), flash memory (FLASH), and mass storage such as hard disks and CD-ROM or CD-RW drives. Each memory type has characteristics that better suit that type to particular applications. For example, DRAM is slower than SRAM but is nonetheless utilized as system memory in most computer systems because DRAM is inexpensive and provides high density storage, thus allowing large amounts of data to be stored relatively cheaply. A memory characteristic that often times determines whether a given type of memory is suitable for a given application is the volatile nature of the storage. Both DRAM and SRAM are volatile forms of data storage, which means the memories require power to retain the stored data. In contrast, mass storage devices such as hard disks and CD drives are nonvolatile storage devices, meaning the devices retain data even when power is removed.
Current mass storage devices are relatively inexpensive and high density, providing reliable long term data storage at relatively cheap. Such mass storage devices are, however, physically large and contain numerous moving parts, which reduces the reliability of the devices. Moreover, existing mass storage devices are relatively slow, which slows the operation of the computer system or other electronic device containing the mass storage device. As a result, other technologies are being developed to provide long term nonvolatile data storage, and, ideally, such technologies would also be fast and cheap enough for use in system memory as well. The use of FLASH, which provides nonvolatile storage, is increasing in popularity in many electronic devices such as digital cameras. While FLASH provides nonvolatile storage, FLASH is too slow for use as system memory and the use of FLASH for mass storage is impractical, due in part to the duration for which the FLASH can reliably store data as well as limits on the number of times data can be written to and read from FLASH.
Due to the nature of existing memory technologies, new technologies are being developed to provide high density, high speed, long term nonvolatile data storage. One such technology that offers promise for both long term mass storage and system memory applications is Magneto-Resistive or Magnetic Random Access Memory (MRAM). FIG. 1 is a functional diagram showing a portion of a conventional MRAM array 100 including a plurality of memory cells 102 arranged in rows and columns. Each memory cell 102 is illustrated functionally as a resistor since the memory cell has either a first or a second resistance depending on a magnetic dipole orientation of the cell, as will be explained in more detail below. Each memory cell 102 in a respective row is coupled to a corresponding word line WL, and each memory cell in a respective column is coupled to a corresponding bit line BL. In FIG. 1, the word lines are designated WL1-3 and the bit lines designated BL1-4, and may hereafter be referred to using either these specific designations or generally as word lines WL and bit lines BL. Each of the memory cells 102 stores information magnetically in the form of an orientation of a magnetic dipole of a material forming the memory cell, with a first orientation of the magnetic dipole corresponding to a logic “1” and a second orientation of the magnetic dipole corresponding to a logic “0.” The orientation of the magnetic dipole of each memory cell 102, in turn, determines a resistance of the cell. Accordingly, each memory cell 102 has a first resistance when the magnetic dipole has the first orientation and a second resistance when the magnetic dipole has the second orientation. By sensing the resistance of each memory cell 102, the orientation of the magnetic dipole and thereby the logic state of the data stored in the memory cell 102 can be determined.
The stored logic state can be detected by measuring the memory cell resistance using Ohm's law. For example, resistance is determined by holding voltage constant across a resistor and measuring, directly or indirectly, the current that flows through the resistor. Note that, for MRAM sensing purposes, the absolute magnitude of resistance need not be known, the inquiry is whether the resistance is greater or less than a value that is intermediate to the logic high and logic low states. Sensing the logic state of an MRAM memory element is difficult because the technology of the MRAM device imposes multiple constraints. In a typical MRAM device, an element in a high resistance state has a resistance of about 950 kΩ. The differential resistance between a logic “1” and a logic “0” is thus about 50 kΩ, or approximately 5% of scale.
Therefore, there is a need for a sensing circuit for a resistance measuring circuit to repeatably and rapidly distinguish resistance values for devices having small signal differentials.